I/O & Interface Automata

I/O & Interface Automata

By Josh Lessard, Josh Taylor, Real Xu

Presenters’ Intro

Presenters’ Problem

Agenda

• Components & Automata

• Interface Automata

• Single-Threaded Interface Automata

• Conclusion

Components & Automata

By Real Xu, [email protected]

User Interface Group, School of Computer Science, University of Waterloo

Objective

• Understand Components & Framework

• Discover relationship between Component-Based Design and Embedded Systems

• Introduction to Component-Based Model of Computation

• Review evolvement

• Understand why we use automata

Components & Framework

• What is component?• subroutines• processes/threads• distributed objects• review of lecture 1• any kind of building block


• What is framework?• Subroutines libraries? No structure• Operating systems? Yes, but weak• CORBA, DCOM? Yes, but confined to

software • Interaction mechanisms? Yes, incorporate

hardware and software• We want: constraints + benefits


Framework

• A class library and policies

• Programming Languages• Operating System• DS & Middleware• Body System• Social/political System

Components

• Already existed methods• Language primitives (a,c) • Single processes/programs• Distributed components• Organs• Companies and

organizations


Framework

1. JavaBeans, COM, CORBA

2. Publish and Subscribe, Linda, JavaSpaces

3. Asynchronous message passing

4. Synchronous message passing

5. Discrete events

6. Continuous time

Interaction Mechanism

1. Unstructured events, no built-in synchronization

2. Event notification, 3. Processes send message through channels

that buffer msgs4. Processes communicate in atomic

instantaneous actions5. Components communicate via signals that

carry events placed in time, which is globally known by all components

6. Processes communicate via continuous-time signals, which are functions on the real numbers.

Component-Based Model of Computation

• Which framework is best? • Your component• States of knowledge• Interaction mechanisms• Specialized, domain dependent!


• Framework problem for embedded system? • We want: as unspecified as possible• Union all? too complex • Choose one? not using all advantages• Use an ADL may get a poor design match• Need a design language, not a descriptive

language!


• The Type System • Ensure software correctness: good! OOP

works, but not for larger structure.• Constrains interface: good!• Ensure compatibility when composing: good!• Static syntax: bad!


• Automata• Use automata to get interface assumptions • Capture dynamic interface properties• Automata give protocols for component

communication • Characterize services that each domain

provides• Use composition and hierarchy of automata

Conclusions

• Components

• Frameworks

• Framework for embedded system

• Type System

• Automata

Interface Automata

By Josh Taylor, [email protected]

What is an Interface Automaton?

• It is an automaton that can be used to determine if two interfaces are compatible

• For simplicity, I will refer to an Interface Automaton as P or Q

An Interface Automaton

P = <VP, VPInit, AI

P, AO

P, AH

P, P> :

• VP is a set of states

• VPInit VP is a set of initial states.

• AIP, A

OP, and AH

P are mutually disjoint sets

of input, output, and internal actions.

Let AP = AIP AO

P AHP

P VP AP VP is a set of steps.

Example Interface Automaton

• Vcomp = {0, 1, 2, 3, 4, 5, 6}

• VcompInit = {0}

• AIcomp = {msg, nack, ack} (?)

• AOcomp = {send, ok, fail} (!)

• AHcomp = (;)

comp = { (0,msg,1), (1,send,2),

(2,ack,5), … }

Properties

• An action aAP is enabled at a state vVP if there is a step (v,a,v)P for some vVP

• AIP(v), AH

P(v), AOP(v) are the subsets of

actions that are enabled at state v• Interface automata are not required to be

input-enabled, that is we do not require AI

P(v) = AIP for all states vVP

• Shared(P,Q) = AP AQ

Composition

• Two interface automata P and Q are composable if

AIP AI

Q = , AOP AO

Q =

AHP AQ = , AH

Q AP = • The composition P||Q of the two interface

automata is obtained by restricting the product P Q to its compatible (non-illegal) states

User and Comp

Consider the product of User and Comp…

Black Box Gives: User Comp

6 is an illegal state. Why? …

Illegal States

• Illegal(User, Comp) = { (v,u) Vuser Vcomp | a Shared(User,Comp)

such that :

( a AOuser(v) and a ! AI

comp(u)) or

(aAOcomp(u) and a !AI

user(v)) }

• In User Comp, the output step (6,fail,0) of Comp has no corresponding input in User

User || Comp

User Comp with illegal states removed, we need an environment so that no input will be generated, that will lead to an illegal state

Legal Environment

• Given two composable interface automata P and Q, a legal environment for the pair (P,Q) is an environment for P Q such that no state in Illegal(P,Q) VE is

reachable in (P Q) E

• The existence of a legal environment for the composition of two interfaces indicates that the interfaces are compatible

Environment for User Comp

• Channel is a legal environment for User Comp because the state (6,u), uVChannel is not reachable

In Closing

• There are algorithms to generate the composition of two interface automata

• Two automata are compatible if there exists a legal environment for the composition

• Interface automata provide a concise and formal notation that parallels the natural way of evolving a component-based design

Single-Threaded Interface Automata

By Josh Lessard, [email protected]

Programming Languages Group, School of Computer Science, University of Waterloo

Introduction

• For uniprocessor systems, interface automata are unnecessarily complex

• Take advantage of single active thread of control

• Single-threaded version of interface automata

• Greatly reduces state space and gives rise to smaller automata

Definition

• A single-threaded interface automaton (STIA) P is an interface automaton that satisfies two conditions:

STIA Condition #1

The set VP of states is partitioned into two disjoint sets VP = VO

P VI

P. The states in VO

P are called running, because only internal and output actions are enabled: for all v VO

P, we have AIP(v) = . The states

in VIP are called waiting, because only input

actions are enabled: for all v VIP, we have

AOP(v) = AH

P(v) = .

STIA Condition #2

All output steps must lead to waiting states: for all (u, a, v) O

P, we have v VIP.

Conversely, only output steps can lead to waiting states: for all v VI

P and all (u, a, v)

P, we have a AOP.

STIA Conditions

• Condition 1 eliminates choice between output/internal actions (ie this automaton advancing thread) and input actions (ie some other automaton advancing thread)

• Running states indicate ownership of the single thread of control; waiting states indicate non-ownership

• Condition 2 ensures that an STIA waits for an input precisely after issuing an output action because if there is only a single thread of control, then each output step relinquishes that thread

Single-Threaded Composition

• Special version of composition for STIAs

• Prunes input actions that occur at states where internal or output actions are also enabled

• Can do this because when in a running state, input for this automaton cannot be produced by other automata

Single-Threaded Composition

Consider two composable STIAs P and Q. The single-threaded composition P|||Q is obtained from P||Q by first removing all steps (v, a, u) I

P||Q for which AOP||Q(v) AH

P||Q

(v) , and then removing all states that become unreachable from Vinit

P||Q.

Example

Example

Invalid input steps removed:

Example

Unreachable states removed:

Conclusion

• Four of the nine states were eliminated (nearly 50%)!!!

• Complexity was greatly reduced• When modelling for uniprocessor systems,

STIAs are a good way to remove clutter from diagrams by doing away with states that are unreachable due to the nature of single threaded systems

Summary of our talk

By Real Xu, [email protected]

User Interface Group, School of Computer Science, University of Waterloo

Summary of our talk

• Why Interface Automata?

• What is Interface Automata?

• How to Use Interface Automata Efficiently?

• Why?- What?- How?

• Future work

Why?- What?- How?

Why?- What?- How?

Why?- What?- How?

• I/O Automata [N. Lynch, M.Tuttle 1989]

• A labelled transition system model• Asynchronous concurrent systems• Actions classified: input (labelled), output,

internal

Why?- What?- How?

Why?- What?- How?

I/O Automata

• What does it do?• Component• Input universal• Pessimistic: compatible if

no error can arise• Based on transition

systems

Interface Automata

• How it can be used?• Interface• Input existential• Optimistic: compatible if

errors can be avoided• Based on game theory

Why?- What?- How?

I/O Automata

• Composition is easy: simply compute the product

• Verification is complex: need to verify that the interface are compatible

Interface Automata

• Composition is complex: requires compatibility check

• Verification is easy: none needed generally

Future Work

• How to adapt to object – orientated code?

• How to model dynamic object creation?

• How to connect to the real software?

Acknowledgement• R.E.Johnson, “Frameworks = (Components + Patterns),” Comm. ACM, Oct. 1997, pp.39-42• T.A.Henzinger, “The Theory of Hybrid Automata,” Proc. 11th Symp. Logic in Computer Science, IEEE CS Press, Los Alamitos,

Calif., 1996, pp278-292• L. de Alfaro, T.A. Henzinger. Interface Automata. In Proceedings of the Joint 8th European Software Engineering Conference and

9th ACM SIGSOFT International Symposium on the Foundations of Software Engineering (ESEC/FSE 01)• Luca de Alfaro and Thomas A. Henzinger. Interface Theories for Component-Based Design. Proceedings of the First International

Workshop on Embedded Software (EMSOFT '01), Lecture Notes in Computer Science 2211, Springer-Verlag, 2001, pp. 148-165. • L. de Alfaro, T.A. Henzinger, R. Jhala. Compositional Methods for Probabilistic Systems. In Proceedings of CONCUR 01:

Concurrency Theory, 12th International Conference, Lectures Notes in Computer Science, Springer-Verlag, 2001.• L. de Alfaro, T.A. Henzinger, R. Majumdar. Symbolic Algorithms for Infinite-State Games. Proceedings of CONCUR 01:

Concurrency Theory, 12th International Conference, Lectures Notes in Computer Science, Springer-Verlag, 2001• L. de Alfaro, T.A. Henzinger, F.Y.C. Mang. The Control of Synchronous Systems. Concurrency Theory, Lectures Notes in

Computer Science, Springer-Verlag, 2001• L. de Alfaro, T.A. Henzinger, F.Y.C. Mang. Detecting Errors Before Reaching Them. Computer-aided Verification, Lectures Notes

in Computer Science 1855, pages 186-201, Springer-Verlag, 2000• Nancy Lynch and Mark Tuttle. An introduction to Input/Output automata. CWI-Quarterly, 2(3):219--246, September 1989.

Centrum voor Wiskunde en Informatica, Amsterdam, The Netherlands• Edward A. Lee, "Overview of the Ptolemy Project," Technical Memorandum UCB/ERL M01/11, University of California, Berkeley,

March 6, 2001• Xiaojun Liu, Jie Liu, Johan Eker, and Edward A. Lee, "Heterogeneous Modeling and Design of Control Systems," to appear in

Software-Enabled Control: Information Technology for Dynamical Systems, T. Samad and G. Balas (eds.), New York City: IEEE Press, 2002.

• Edward A. Lee, “What’s Ahead for Embedded Software?” IEEE 33:18-26, 2000• Edward A. Lee, Y. Xiong. System-level Types for Component-based Design. Technical Memorandum UCB/ERL M00/8,

Electronics Research Lab, University of California, Berkeley, 2000

Thanks for involvements and questions and answers!

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I/O & Interface Automata